1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an LCD having dual light units.
2. Description of the Related Art
In general, LCDs are flat panel display devices having a relatively small size, slim profile, and low power consumption. Accordingly, LCDs are commonly used in mobile computers, such as notebook computers, office automation machines, and audio/video machines.
The LCD displays images by manipulating the transmission of light through a liquid crystal material by controlling an electric field induced to the liquid crystal material. The LCD does not necessarily emit the light by itself, but makes use of an external light source. Such a technique is in contrast to other display devices, such as electro-luminescences (ELs), cathode ray tubes (CRTs), and light emitting diodes (LEDs), which emit light on their own.
In general, the LCDs can be classified into two different categories: transmission LCDs and reflective LCDs. The transmission LCD includes a liquid crystal panel having a liquid crystal layer interposed between two substrates. In addition, the transmission-type LCD includes a back light unit that supplies the light to the liquid crystal panel. However, it is difficult to manufacture the transmission LCDs having slim profiles and light weight due to the volume and the weight of the back light unit. In addition, the back light unit requires large amounts of electrical power.
Conversely, the reflective LCDs are not separately provided with light sources, but display images depending on natural (ambient) light conditions. Thus, because the reflective LCDs do not require any additional light sources, the reflective LCDs consume small amounts of electrical power and can be widely employed in mobile display devices, such as electronic notes and personal digital assistants (PDAs). However, when the ambient light is not sufficient, i.e. at night, a brightness level of the reflective LCDs is lowered, whereby the information displayed cannot be read. Thus, to overcome this problem, a method of displaying images under dark conditions includes installing a front light unit in the reflective LCDs.
FIG. 1 is a perspective schematic diagram illustrating a reflective LCD using a front light unit according to the related art, and FIG. 2 is a cross sectional view illustrating the reflective LCD of FIG. 1 using a front light unit according to the related art. In FIGS. 1 and 2, an active LCD 100 includes a reflective liquid crystal panel 120 and a front light unit 110 positioned on the reflective liquid crystal panel 120 to provide light. The reflective liquid crystal panel 120 is provided with a first substrate 121 and a second substrate 122, wherein a diffusing reflective electrode 123 is formed on the second substrate 122. The diffusing reflective electrode 123 reflects ambient light supplied from an upper surface of the reflective liquid crystal panel 120 or reflects incident light emitted from the front light unit 110.
The front light unit 110 includes a light source 111, a light guide plate 112, and a reflective mirror 113, wherein the light source 111 generates light. The light guide plate 112 projects the light onto a display surface of the reflective liquid crystal panel 120. The reflective mirror 113 reflects the light generated from the light source 111 to the light guide plate 112.
In FIG. 2, the upper surface of the light guide plate 112 is formed having a prismatic configuration such that the light supplied from the light source 111 is reflected by an upper surface and a lower surface of the light guide plate 112. Then, the light supplied to the light guide plate 112 is supplied along a direction perpendicular to the reflective liquid crystal panel 120 positioned below the light guide plate 112. Next, the light supplied vertically to the reflective liquid crystal panel 120 is reflected by a reflective electrode 123 of the reflective liquid crystal panel 120, and travels upward over the light guide plate 110, thereby displaying an image.